cli: provide error context for ssh-agent connect — heartwood

Provide more context to the ssh-agent error for easier debugging when rad auth. fails.

 use std::ops::Not as _;
 use std::str::FromStr;
 use anyhow::anyhow;
 use anyhow::{anyhow, Context};
 use radicle::crypto::ssh;
 use radicle::crypto::ssh::Passphrase;

             Err(e) if e.is_not_running() => {
                 agent = false;
             }
             Err(e) => Err(e)?,
             Err(e) => Err(e).context("failed to connect to ssh-agent")?,
         }
     }